As the size of functional elements in integrated circuits decreases, complexity and interconnectivity increases. To accommodate the growing demand of interconnections in modern integrated circuits, on-chip interconnections have been developed. Such interconnections generally consist of multiple layers of metallic conductor lines embedded in low dielectric constant materials. The dielectric constant in such materials has a very important influence on the performance of an integrated circuit. Materials having low dielectric constants (i.e., below 2.2) are desirable because they allow faster signal velocity and shorter cycle times. Moreover, lowering of the dielectric constant reduces capacitive effects, leading often to less cross talk between conductor lines and lower voltages to drive integrated circuits.
One way of achieving low dielectric constants in the insulator materials is to employ materials with inherently low dielectric constants. Generally, two different classes of low dielectric constant materials have been employed in recent years--inorganic oxides and organic polymers. Inorganic oxides often have dielectric constants between 3 and 4, and have been widely used in interconnects with design rules larger than 0.25 .mu.m. As the dimensions of the interconnects shrink, materials with a lower dielectric constant become more desirable. Organic polymers have shown many advantageous properties including high thermal stability, ease of processing, low stress/TCE, low dielectric constant and high resistance, and are therefore considered as alternative low dielectric constant polymers for the 0.18 .mu.m and subsequent generations of increasingly smaller dimensions.
With respect to other properties, desirable dielectrics should also be free from moisture and out-gassing problems, have suitable adhesive and gap-filling qualities, and have suitable dimensional stability towards thermal cycling, etching, and CMP processes (i.e., chemical mechanical polishing). Suitable dielectrics should also have Tg values (glass transition temperatures) of at least 300.degree. C., and preferably 500.degree. C. or more.
Extrapolating the needs to design rules of 0.07 .mu.m and below suggests a strong need for materials having dielectric constant lower than 2.2. This has led to the development of dielectric materials with designed-in nanoporosity. Since air has a dielectric constant of about 1.0, a major goal is to reduce the dielectric constant of nanoporous materials down towards a theoretical limit of 1.
Several approaches are known in the art for fabricating nanoporous materials. In one approach, a thermostable polymer is blended with a thermolabile (thermally decomposable) polymer. The blended mixture is then crosslinked and the thermolabile portion thermolyzed. Examples are set forth in U.S. Pat. No. 5,776,990 to Hedrick et al. In another approach, thermolabile blocks and thermostable blocks alternate in a single block copolymer. The block copolymer is then heated to thermolyze the thermolabile blocks. In a third approach, thermostable blocks and thermostable blocks carrying thermolabile portions are mixed and polymerized to yield a copolymer. The copolymer is subsequently heated to thermolyze the thermolabile blocks. In yet a fourth approach, small hollow glass spheres are introduced into a material. Examples are given in U.S. Pat. No. 5,458,709 to Karnezaki and U.S. Pat. No. 5,593,526 to Yokouchi.
Regardless of the approach used to introduce the voids, structural problems are frequently encountered in fabricating nanoporous materials. Among other things, increasing the porosity beyond a critical extent (generally about 30% in the known nanoporous materials) tends to cause the porous materials to collapse. Collapse can be prevented to some degree by adding crosslinking additives that couple thermostable portions with other thermostable portions, thereby producing a more rigid network. electrical properties of the polymer might be disadvantageously altered, especially when relatively complex crosslinking functionalities are employed.
In summary, various methods are known to crosslink polymers in nanoporous materials. However, current methods frequently complicate the synthesis of monomers, or tend to interfere with either the polymerization reaction or physicochemical properties of nanoporous materials. Surprisingly, despite great efforts to improve various properties in nanoporous materials, and considerable work in improving crosslinking in nanoporous materials, there is no general method for crosslinking (a) without relying on exogenous crosslinking molecules, and (b) without adding pendent functionalities to the monomers. Therefore, there is still a need for methods and compositions that circumvent these limitations.